1. Field of the Invention
This invention generally relates to antennas, and more specifically to an ultra-wideband magnetic antenna.
2. Related Art
Recent advances in communications technology have enabled communication and radar systems to provide ultra-wideband channels. Among the numerous benefits of ultra-wideband channels are increased channelization, resistance to jamming and low probability of detection.
The benefits of ultra-wideband systems have been demonstrated in part by an emerging, revolutionary ultra-wideband technology called impulse radio communications systems (hereinafter called impulse radio). Impulse radio was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989) and U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. patent application Ser. No. 07/368,831 (filed Jun. 20, 1989) all to Larry W. Fullerton. These patent documents are incorporated herein by reference.
Basic impulse radio transmitters emit short Gaussian monocycle pulses with tightly controlled pulse-to-pulse intervals. Impulse radio systems can use pulse position modulation, which is a form of time modulation in which the value of each instantaneous sample of a modulating signal is caused to modulate the position in time of a pulse.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Generally, spread spectrum systems make use of pseudo-random codes to spread the normally narrow band information signal over a relatively wide band of frequencies. A spread spectrum receiver correlates these signals to retrieve the original information signal. Unlike spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code is used for channelization, energy smoothing in the frequency domain and jamming resistance.
The impulse radio receiver is a homodyne receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The baseband signal is the basic information channel for the basic impulse radio communications system, and is also referred to as the information bandwidth. The data rate of the impulse radio transmission is only a fraction of the periodic timing signal used as a time base. Each data bit time position modulates many pulses of the periodic timing signal. This yields a modulated, coded timing signal that comprises a train of identical pulses for each single data bit. The cross correlator of the impulse radio receiver integrates multiple pulses to recover the transmitted information.
Ultra-wideband communications systems, such as the impulse radio, poses very substantial requirements on antennas. Many antennas are highly resonant operating over bandwidths of only a few percent. Such "tuned," narrow bandwidth antennas may be entirely satisfactory or even desirable for single frequency or narrow band applications. In many situations, however, wider bandwidths may be required.
Traditionally when one made any substantial change in frequency, it became necessary to choose a different antenna or an antenna of different dimensions. This is not to say that wide band antennas do not, in general, exist. The volcano smoke unipole antenna and the twin Alpine horn antenna are examples of basic wideband antennas. The gradual, smooth transition from coaxial or twin line to a radiating structure can provide an almost constant input impedance over wide bandwidths. The high-frequency limit of the Alpine horn antenna may be said to occur when the transmission-line spacing d&gt;.lambda./10 and the low-frequency limit when the open end spacing D&lt;.lambda./2. These antennas, however, fail to meet the obvious goal of transmitting sufficiently short bursts, e.g., Gaussian monocycle pulses. Also, they are large, and thus impractical for most common uses.
A broadband antenna, called conformal reverse bicone antenna (hereinafter referred to as the bicone antenna) suitable for impulse radio was described in U.S. Pat. No. 5,363,108 to Larry Fullerton. FIG. 1 illustrates a front view of a bicone antenna 100. The bicone antenna 100 radiates burst signals from impulses having a stepped voltage change occurring in one nanosecond or less. The bicone antenna 100 is basically a broadband dipole antenna having a pair of triangular shaped elements 104 and 108 with closely adjacent bases. The base and the height of each element is approximately equal to a quarter wavelength (.lambda./4, where .lambda. is a wavelength) of an electromagnetic wave having a selected frequency. For example, in a bicone antenna designed to have a center frequency of 650 MHz, the base of each element is approximately four and a half inches (i.e., .lambda./4=four and a half inches) and the height of each element is approximately the same.
Although, the bicone antenna 100 performs satisfactorily for impulse radios, further improvement is still desired. One area in which improvement is desired is reduction of unbalanced currents on the feed cable, e.g., a coaxial type cable, of a wide-band antenna. Generally, impulse radios operate at extremely high frequencies, typically at 1 GHz or higher. At such high frequencies, currents are excited on the outer feed cable because of the fields generated between the center conductor and the outside conductor. These currents are unbalanced having poorly controlled phase, thereby resulting in distorted ultra-wideband pulses. Such distorted ultra-wideband pulses have low frequency emissions that degrade detectability and cause problems in terms of frequency allocation.
Generally, unbalanced currents on feed cables are filtered by balun transformers or RF chokes. However, at frequencies of 1 GHz or higher, it is extremely difficult to make balun transformers or RF chokes, due to degraded performance of ferrite materials. Furthermore, balun transformers suitable for use in ultra-wideband systems are difficult to design. As a result, unbalanced currents remain a concern in the design of ultra wide-band antennas.
A second area where improvement is desired is the isolation of a transmitter from a receiver in an ultra-wideband communications system. Because the bicone antenna 100 generates a field pattern that is omni-directional in the azimuth, it is difficult to isolate a transmitter from a receiver. Additionally, isolation between antennas is desired where a plurality of antennas are arranged in an array. In an array system, isolation significantly reduces loading of one element by an adjacent element.
For these reasons, many in the ultra-wideband communications environment has recognized a need for an improved antenna that provides a significant reduction in unbalanced currents in feed cables. There is also a need for an antenna suitable for ultra-wideband communication systems that provides improved isolation between transmitters and receivers as well as between antenna elements in an array system.